All the molds I have made, have been pretty basic. No complex shapes, my next project is shaped like a crescent/moon and is only 2 inches wide. I'm trying to find the best materials to support the part or fill the gap. I attach a pic done in paint as an example. Not for sure if I should use expanding foam, foam block or poster board to fill the gap.

If all it needs to do is support the object then use whatever you like that's easy. If it were me, I'd use a block of plasticine or something cheap to fill the bulk of the space, and then filleting wax on the edges to get a nice smooth radius.

That said, why not bend the polypropylene sheet to the same shape as your part and make it much easier for yourself? Polypropylene bends very easily with a bit of heat, or you could switch to core flute which is also easy to curve to that shape.